Repeated attempts have been made to use the principles of nuclear magnetic resonance by means of logging tools lowered through boreholes in oil exploration over the past several decades. It is recognized that particles of a formation having magnetic spin, for example atomic nuclei, protons, or electrons, have tendencies to align with a magnetic field which is imposed on the formation. Such a magnetic field may be naturally generated, as is the case with the earth's magnetic field (B.sub.E) which has an intensity of approximately 0.5 gauss in areas of the globe where boreholes are typically drilled. Any given particle in a formation is additionally influenced by localized magnetic fields associated with nearby magnetic particles, other paramagnetic materials, and the layer of ions which typically line pore walls of certain types of formations such as shales. These localized fields tend to be inhomogeneous, while the earth's magnetic field is relatively homogeneous.
Useful apparatus have been provided which produce a static and substantially homogeneous magnetic field focussed into a formation on one side of a logging tool. By directing and configuring the combined magnetic fields of a configuration of magnets, a region, remote from the configuration of magnets, is produced wherein the spatial field gradient substantially vanishes, thereby insuring that the field is highly homogeneous throughout that region. In a preferred form, the magnets are mounted within a metallic skid or logging pad, the static magnetic field is directed through the face of the pad into an adjacent formation, and the region of substantially homogeneous field is situated in a volume of formation behind the mudcake layer which typically lines borehole walls. A homogeneous magnetic field one thousand times stronger than the earth's magnetic field can be thus imposed, or "focused", on a volume of formation in situ. Reference may be had to U.S. Pat. No. 4,933,638 issued Jun. 12, 1990 to Kenyon, et al for details of such an apparatus, which patent is incorporated herein by reference.
In the Kenyon et al patent an RF antenna is mounted on the outside of the structure of the pad so that the pad serves as a natural shield against any signals which may be generated by resonant conditions behind the pad, particularly those potentially strong resonance signals from borehole fluid. In the preferred form, the antenna is configured to focus its signals radially outwardly from the pad face, into the volume of formation having the homogeneous field, thereby additionally reducing the distortion of measured signals from borehole effects.
While the prior logging tools of Kenyon, et al have been capable of determining formation characteristics with sufficient accuracy and dependability, it has been found useful to improve their performance and accuracy in view of the inherent difficulties of making NMR measurements in boreholes.
Accordingly, it is an object of the present invention to provide improved apparatus and methods for determining nuclear magnetic characteristics of earth formations more accurately and more dependably.
It is an additional object of the invention to provide apparatus and methods to determine the nuclear magnetic relaxation time, the free fluid porosity, permeability and related pore fluid characteristics of earth formations traversed by a borehole at two different depths of investigation into the formations surrounding the borehole.
It is also an object of the invention to provide borehole apparatus for measuring magnetic resonance in which all pulse parameters are controlled so as to operate repeatedly in borehole conditions with high reliability and accuracy.
It is a further object of the invention to provide a magnet configuration for NMR measurements in a logging borehole which is easily and conveniently tested, calibrated and compensated for temperature variations.
It is also an object of the invention to provide a magnetic resonance logging tool which directly measures the transverse and longitudinal relaxation times of formations traversed by a borehole.
In another aspect of the invention, it is an object to provide improved methods for determining the permeability and like parameters from measured longitudinal "T.sub.1 " and transverse "T.sub.2 " relaxation times signals of NMR logging tools.
It is also an object of the invention to provide improved methods for determining the nuclear magnetic relaxation times of a measured population of particles in earth formations surrounding a borehole.